As an image forming apparatus such as a printer, a facsimile, a copier, a plotter, and a multifunction peripheral of these apparatuses, for example, a liquid discharge recording type image forming apparatus using a recording head for discharging ink droplets has been used. As such a liquid discharge recording type image forming apparatus, an inkjet recording apparatus and the like have been known. The liquid discharge recording type image forming apparatus discharges ink droplets from a recording head onto transfer paper (transfer paper is not limited to paper but includes OHP (overhead projector) and anything to which ink droplets and other liquid can attach, which is also referred to as a medium to be recorded, a recording medium, recording paper, a recording sheet, and the like) to form an image (recording and printing an image and text, and imaging mean the same as forming an image). As the liquid discharge recording type image forming apparatus, there are a serial type image forming apparatus having a recording head that discharges liquid droplets to form an image while moving in a horizontal scanning direction, and a line type image forming apparatus having a line type head (line head) which discharges liquid droplets to form an image in a stationary state.
In the present invention, an “image forming apparatus” means an apparatus that performs an image formation by discharging liquid onto a medium such as paper, a string, a fiber, a cloth, leather, a metal, plastic, glass, wood, and ceramics. Moreover, the “image formation” means to attach an image having a meaning such as text and a figure to the medium, and in addition, to attach an image having no meaning such as a pattern to the medium (simply to land liquid droplets on the medium). Further, “ink” is not limited to what is called ink, but is used as a collective term for all liquid capable of being used for image formation, such as those called a recording solution, a fixing processing solution, and liquid. For example, ink includes a DNA sample, a resist, a pattern material, and the like.
As such an image forming apparatus, as described above, there has been known a line type image forming apparatus having a line type recording head in which a nozzle array corresponding to a width of paper to be transferred is arranged. Here, for example, a line type recording head in which plural short heads each having an array of nozzles (nozzle array) for discharging liquid droplets are arranged is used. The plural short heads are arranged in a direction of the nozzle arrays in this line type recording head (this recording head is called a “head module” and the plural head modules arranged together are called a “recording head unit” or a “multi-array head”).
In such a head module, the plural heads are required to be positioned with a high precision when arranged in an array. Moreover, in a recording head unit in which plural head modules are arranged, each head module is required to be positioned with a high precision.
In view of the above circumstances, for example, Patent Document 1 discloses a configuration in which a recording head is fixed in a module base by adjusting its position by using a nozzle array direction position adjusting mechanism (taper pin) that also regulates a discharge direction position, and using an inclination adjusting mechanism (eccentric cam), and a configuration where a similar adjustment mechanism is applied for a positional adjustment to fix a head module by a screw, to provide a recording head position adjustment method capable of realizing recording of a high quality image by a method where a positional deviation of an ink in the ejection direction ejected from a nozzle of a recording head is prevented when the adjustment of the position of the recording head is carried out, and the recording head is fixed on the position in the preliminary fixing.
[Patent Document 1] Japanese Patent Application Publication No. 2006-188013
Further, Patent Document 2 discloses a configuration to reduce a positional change of a discharging outlet caused by thermal expansion of a thermal type head serving as a long head.
[Patent Document 2] Japanese Patent Application Publication No. 2004-098473
In the case of forming a line head by using a head module in which plural heads are arranged, in general, one head module is formed by arranging two arrays of plural heads in a staggered manner to discharge one line of liquid droplets of the same color. A recording head unit is formed by positioning and fixing each head module with respect to a predetermined reference.
However, in the recording head unit in which plural head modules each having plural heads are arranged, it is difficult to replace each head. Therefore, it is preferable that the replacement can be performed on a head module by head module basis, that is, on a head module level.
However, for example, in the case where one head module is formed by arranging plural (ten) heads for discharging liquid droplets of the same color in a staggered manner, and one of the heads having a defect is required to be replaced, the remaining non-defective nine heads, ink and electric connecting components for the ten heads, and the like have to be thrown away because of the one defective head. As a result, high cost is required for the replacement.
In view of this, the head module, in which plural heads for discharging liquid droplets of the same color are arranged in a staggered manner, is divided into two so that one head module is formed of one array of heads. Consequently, the number of heads to be replaced, and the ink and electric connecting components attached to the heads to be replaced are reduced to half, thereby cost required for the replacement can be reduced.
In the case where the plural heads for discharging ink of the same color are divided to be arranged on two head modules, however; the two head modules are required to be positioned with a high precision on a base member (referred to as an array base member) that is commonly used for the two head modules. That is, when all of the plural heads for discharging the liquid droplets of the same color are arranged on one base member (this is referred to as a “module base member”), each of the plural heads can be positioned with a high precision by positioning by utilizing an image processing method and the like. However, when the plural heads are divided and arranged in the two head modules, such a positioning by the image processing method cannot be performed in order to make it easier to perform replacement in the head module level, that is, on the head module by head module basis.
Furthermore, even in the case of using a line type head formed of one head having a nozzle array of one line instead of using a head module in which plural heads are arranged, when plural line heads are arranged to form a recording head unit, it is important to position the two line heads on the common array base member with a high precision to form a high quality image, in such a case where at least two line heads arranged in a paper transfer direction discharge liquid droplets of the same color to enhance a pixel density.
In the conventional configuration, however, when two or more head modules or heads are set to discharge liquid droplets of the same color and each of the head modules or heads is replaced (that is, when replacement is performed in the head module level or the head level), reproducibility of the positional precision has been poor.